Field of the Invention
The present invention relates to pigments with photocatalytic activity, a method for the production thereof and a coating agent.
Description of Related Art
Photocatalytic self-cleaning refers to a property of surfaces which have been coated e.g. with nanoparticles of titanium dioxide (TiO2). Organic materials on the surface are broken down by irradiation with (sun)light. The surfaces stay clean and have an antimicrobial action.
The principle of the photocatalytic effect is based on the semi-conductor properties of titanium dioxide (TiO2). The impingement of light produces electron-hole pairs, provided that the energy of the photons is greater than the band gap Eg (internal photoelectric effect). The electrons or holes can diffuse in the titanium dioxide on the surface and produce radicals there which lead to the breakdown of organic substances. In particular the holes have a highly oxidative action, e.g. OH radicals can be formed in this way even from water. Organic substances are broken down by the formed radicals; in many cases end-products are CO2 and water.
Up to now, self-cleaning surfaces, e.g. on motor vehicle mirrors, self-cleaning or antimicrobially active tiles, as well as photocatalytically active emulsion paints, paving stones, flagstones, exposed concrete, roofing tiles, papers, as well as water treatment and air purification have been described as fields of use for this photocatalytic effect.
In the state of the art, primarily TiO2 particles with widely different degrees of fineness and in particular nanoparticles based on an anatase-TiO2 basis are described for the photocatalysis. For another thing, pearlescent pigments covered with TiO2 (anatase) are also mentioned. WO 2008/034510 A2 relates to a photocatalytically active coating for surfaces which contains commercially available interference pigments as photocatalytically active material.
If TiO2 nanoparticles are used as photocatalytically active material, the possible dangers that accompany them for manufacturers and handlers are also not to be underestimated. It is known that nanoparticulate titanium dioxide can be absorbed by humans both through the lungs and through the skin or the alimentary canal, and can lead to accumulations there. When used in particular on the external surfaces of components of all types, the transfer of substantial amounts of nanoparticles into the groundwater is also not to be ruled out.
When TiO2 particles or TiO2 nanoparticles are used, undesired color phenomena, such as e.g. clouding, scattering or whitening, can occur.
When commercially available pearlescent pigments are used, a significant gloss effect is additionally very often to be observed. Likewise, commercially available pearlescent pigments, depending on the TiO2 proportion in the coating, produce a color effect, so-called interference colors.
A significant disadvantage of TiO2 particles is represented by the problem that the intended photoactivity and the accompanying tendency of titanium dioxide to form radicals cannot distinguish between “friend” and “foe”. The TiO2 particles cannot differentiate whether organic components in their surroundings are now to be decomposed or preserved. In other words, as a rule, organic binder systems which come into consideration as matrix or carrier for photocatalytically active TiO2 particles are also radically decomposed. This effect is, naturally, not desired since an achievable action would then only be available in the short-term. Therefore most coatings with photocatalytic activity are currently limited to insensitive, inorganic binder matrices, or are only effective for a short time.
Macroscopically, this expresses itself for example as a strong chalking and makes the use of organic binders almost impossible at present.